A light emitting element using an organic compound is an element in which a layer including the organic compound (or organic compound film) emits light by applying an electric field. A structure in which a layer including a luminescent organic compound (light emitting layer) is interposed between a pair of electrodes is a basic structure of the light emitting element. By applying voltage to this element, electrons and holes are transported to the light emitting layer respectively from the pair of electrodes, and current flows. Then, the luminescent organic compound forms an excited state by re-combining those carriers (electrons and holes), and light is emitted when the excited state returns to a grand state.
In such a light emitting element, an organic compound film is usually formed to have a thin thickness below 1 μm. Further, since such a light emitting element is a self-luminous element in which the organic compound film itself emits light, a backlight used for conventional liquid crystal displays is unnecessary. Accordingly, the light emitting element has a great advantage that it can be manufactured to have an ultra thin film thickness and lightweight. Further, in the case of an organic compound film of approximately 100 to 200 nm, time between the injection of carriers and recombination thereof is about several ten nanoseconds considering the carrier mobility of the organic compound film. The time required for light-emission is about 1μ second or less, even if including a process from the recombination of carriers to the emission of light. Thus, an extremely high response speed is also one of the features thereof. In addition, since such a light emitting element is a carrier injection type light emitting element, driving by a direct current voltage is possible, therefore, noise is hardly caused. With respect to a driving voltage, the organic compound film is formed to be a uniform ultra thin film having a thickness of approximately 100 nm, and a material for an electrode is selected so as to reduce a carrier injection barrier to the organic compound film. Further, a hetero structure (here, two-layer structure) is introduced to achieve a sufficient luminance of 100 cd/m2 at 5.5 V (for example, see C. W. Tang and the other, Applied Physics Letters, vol. 51, No. 12, 913-915 (1987)).
In addition to the element characteristics such as the thin thickness and lightweight, the high response speed, and direct-current low-voltage driving, it can also be regarded as one of the great advantages that the light emitting element using an organic compound has wide variation of emission colors. The factor is the multiplicity of the organic compound itself. That is, flexibility that materials of the various emission colors can be developed by molecule design (for example, introducing a substituent) and the like, produces richness in colors. It can be said that the biggest application field of the light emitting element utilizing the richness in colors is a full color flat panel display. Many organic compounds that can emit three primary colors of light, that is, red, green, and blue, exist. Therefore, by patterning thereof, full color can be easily accomplished.
The element characteristics such as the thin thickness and lightweight, the high response speed, and direct-current low-voltage driving as described above can also be regarded as the characteristics that are suitable for a flat panel display. In recent years, as an attempt to further improve luminous efficiency, it is proposed to use not a fluorescent material but a phosphorescent material. In a light emitting element using the organic compound, light is emitted when a molecular exciton returns to the ground state. For the light emission element, luminescence from a singlet excitation state (S*) (fluorescence) and the luminescence from a triplet excitation state (T*) (phosphorescence) are possible. In the case where the fluorescent material is used, only luminescence from S* (fluorescence) contributes to the light emission.
However, it is believed that a statistical generation ratio of S* to T* in the light emitting element is S*:T*=1:3 (for example, see Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Organic Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, P. 31 (1993)). Accordingly, the theoretical limit of the internal quantum efficiency (the ratio of generating photons to injected carrier) in a light emitting element using the fluorescent material is believed to be 25% on the ground of S*:T*=1:3. In other words, in the case of a light emitting element using the fluorescent material, at least 75% of the injected carriers are wasted.
On contrary, it is considered that luminous efficiency is improved (simply by 3 to 4 times) if luminescence from T*, that is, phosphorescence can be utilized. However, in the case of a usual organic compound, luminescence from T* (phosphorescence) is not observed at a room temperature, and normally, only luminescence from S* (fluorescence) is observed. This is because the ground state of an organic compound is normally a singlet ground state (S0), and thus, T*→S0 transition is a forbidden transition and S*→S0 transition is an allowed transition. However, the announcements of a light emitting element that is capable of converting energy released in returning to the ground state from T* (hereinafter referred to as “triplet excitation energy”) into luminescence have been given one after another in recent years, and the highness of the luminous efficiency has attracted attention (for example, see Tetsuo TSUTSUI, and eight others, Japanese Journal of Applied Physics, vol. 38, L1502-L1504 (1999)).
A metal complex with iridium as a central metal (hereinafter referred to as “iridium complex”) is used as a luminescent material in this reference. It can be said that introducing a third transition series element as the central metal is a feature. These are materials capable of converting a triplet excitation state into luminescence (material capable of emitting phosphorescence) at a room temperature. As shown this reference, the light emitting element using the substance capable of emitting phosphorescence can accomplish higher internal quantum efficiency than conventionally prior art. Then, as the internal quantum efficiency becomes higher, the luminous efficiency ([lm/W]) is improved.
However, as for the Ir complex disclosed in this reference only a complex exhibiting green emission of light is disclosed. Accordingly, at present, a development of a substance that can emit phosphorescence of various colors is desired.